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我正在尝试实现 Vigenere Cipher，但在尝试通过密钥循环时遇到问题。首先，所有的小功能，虽然，我认为问题不存在。

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>

#define ALPHANUMERIC "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789" /* all alphabetic characters at their corresponding indices */

char* squeeze(char[], int);
char* formatString(char[]);
int findindex(char c);

/* squeeze: remove all occurences of c in s */
char* squeeze(char s[], int c) 
{

    int i, j;

    for (i = j = 0; s[i] != '\0'; i++)
        if (s[i] != c)
            s[j++] = s[i];
    s[j] = '\0';
    // printf("%s\n", s);
    return s;
}

/*  formatString: remove all white spaces and special 
    characters and convert all letters to upper case */
char* formatString(char input[]) 
{
    int i;

    input = squeeze(input, ' ');

    for (i = 0; input[i] != '\0'; i++)
    {
        if (!isalnum(input[i]))
        {
            input = squeeze(input, input[i]);
            --i;
        }
        if (islower(input[i]))
            input[i] = toupper(input[i]);
    }
    // printf("%s\n", input);
    return input;
}

/* findindex: returns the predefined index of a given character */
int findindex(char c)
{
    int i;

    if (isdigit(c))
        return 26 + c - '0';

    for (i = 0; ALPHANUMERIC[i] != '\0'; i++)
        if (c == ALPHANUMERIC[i])
            return i;
    return -1;
}

这是密码功能。澄清一下，我的想法是循环遍历 for 循环中的文本字符串，并使用 j 计数器在到达代码字末尾时通过将其设置为零来循环代码字。所有额外的 printf-s 都用于调试目的。

void cipher(char codeword[], char text[])
{
    int i, j;
    char newword[strlen(text)];
codeword = formatString(codeword);
text = formatString(text);

    printf("codeword = %s, text = %s\n", codeword, text );
    j = -1;
    for (i = 0; i < strlen(text); i++)
    {
        j++;

        printf("text[i] = %c, %d, codeword = %c, %d\n", text[i], findindex(text[i]), codeword[i], findindex(codeword[i]));

        newword[i] = ALPHANUMERIC[(findindex(text[i]) + findindex(codeword[j])) % 36];

        printf("i = %d, j = %d\n", i, j);

        if (j == strlen(codeword) - 1)
            j = -1;
    }

    printf("%s\n", newword);
}

所以，看看代码，一切似乎都很好，直到我运行它。这是输入“ivcho”、“ivayloivayloo”的输出：

codeword = IVCHO, text = IVAYLOIVAYLOO
text[i] = I, 8, codeword = I, 8
i = 0, j = 0
text[i] = V, 21, codeword = V, 21
i = 1, j = 1
text[i] = A, 0, codeword = C, 2
i = 2, j = 2
text[i] = Y, 24, codeword = H, 7
i = 3, j = 3
text[i] = L, 11, codeword = O, 14
i = 4, j = 4
text[i] = O, 14, codeword = , -1
i = 5, j = 0
text[i] = I, 8, codeword = , -1
i = 6, j = 1
text[i] = V, 21, codeword = , -1
i = 7, j = 2
text[i] = A, 0, codeword = , -1
i = 8, j = 3
text[i] = Y, 24, codeword = G, 6
i = 9, j = 4
text[i] = L, 11, codeword = C, 2
i = 10, j = 0
text[i] = O, 14, codeword = C, 2
i = 11, j = 1
text[i] = O, 14, codeword = :, -1
i = 12, j = 2
QGC5ZW3XHCT9Q

当计数器 (j) 设置为零时，它就不会重新开始，我不知道为什么。

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